In an RNC (Radio Network Controller) with which the present invention is concerned, as shown in FIG. 1, an IMT (International Mobile Telecommunications) 2000 system which is a third-generation mobile communication system controls a plurality of base stations (Node B) 2, 3 to manage a plurality of cell information under one base station.
For making a radio connection to UE (User Equipment), RNC 1 selects a cell according to a cell selection algorithm that takes into consideration various parameters such as UE capability, service sought by the UE, cells which can provide a service to the UE, a high-speed service providing situation, a cell load calculated from the number of UEs of each cell and a service load executed by the UE, and the congestion state of base stations 2, 3. RNC 1 then confirms whether the selected cell can accept the newly called UE or not and assigns the UE to the cell.
RNC 1 operates according to the cell selection algorithm shown below. The cell selection algorithm will be described below with reference to FIG. 1.
As disclosed in JP2006-229384A, for example, RNC 1 covers a mixture of an E-DCH [Enhanced uplink DCH (Dedicated Channel)]/HS-DSCH (High Speed Downlink Shared Channel) service unprovidable cell, an HS-DSCH cell, and an E-DCH/HS-DSCH cell which have different frequencies with the same antenna, and is triggered by the moments shown below to carry out Local Control to select a cell in an environment wherein a mixture of E-DCH/HS-DSCH service unexecutable UE 4, HS-DSCH capable UE 5, and E-DCH/HS-DSCH capable UE 6 is present in the same area.
The moments are:
1. when RNC 1 receives a first RRC (Radio Resource Control) Connection Request message: and
2. when RNC 1 changes channel types from a CTS FACH (Forward Access Channel) to a DCH upon the reception of a traffic increase report from the UE or upon detection of a transfer buffer increase in the RNC while connecting a packet service via a shared channel.
When RNC 1 executes the cell selection algorithm, RNC 1 does not recognize the actual radio environment far ahead. Therefore, when RNC 1 assigns a cell which is different from the cell that the UE has camped on, RNC 1 does not receive a response from the UE indicating that it has established a radio connection a the new cell. Therefore, the call connection completion ratio is lowered.
The above problem is caused by a phenomenon called cell breathing wherein a cell that is capable of high speed data services has its cell size becoming larger or smaller with time due to the very large number of UEs which execute high speed data services and the degree of interference with adjacent cells.
For example, when UE seeking a high speed data service camps on a cell that is not capable of providing a high speed data service and sends an RRC Connection Request message, cell breathing occurs as shown in FIG. 2, and RNC 1 may select a high speed data providing cell having a small cell size according to the cell selection algorithm.
In such a case, RNC 1 selects the cell whose cell size is small due to the cell breathing shown in FIG. 2 and sends an RRC Connection Setup message, and is unable to receive an RRC Connection Setup Complete response from the UE.
Even when the UE sends an RRC Connection Request message again, RNC 1 selects the same cell according to the same cell selection algorithm. As RNC 1 fails to make a radio connection, no radio connection will be completed, and the call connection completion ratio will be greatly lowered, in the same manner as described above.
In this case, even when there is no UE response from a high speed data providing cell and the UE sends an RRC Connection Request message again, RNC 1, 1a similarly selects the same cell suffering cell breathing. Therefore, the call connection completion ratio will be greatly lowered.
The above cell selection algorithm with which the present invention is concerned is problematic in that when the size of a cell selected by the cell selection algorithm is temporarily reduced by cell breathing caused by the number of connected UEs and interference with adjacent cells, the RNC fails to make a radio connection, and the latest information indicating that there is a cell suffering radio connection failure is not stored or used.
The cell selection algorithm with which the present invention is concerned is also problematic in that the same cell is selected according to the cell selection algorithm upon the resending of an RRC Connection Request from the EU, the periodic reception of a traffic increase report from the UE, or the periodic detection of a transfer buffer increase in the RNC, so that the RNC fails to make a radio connection and the call connection completion ratio will be lowered, in the same manner as described above.